The present invention relates generally to a micro-pixel ultraviolet light-emitting device and method of manufacturing a light-emitting device.
Group III nitride compound semiconductors such as, for instance, gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN) (hereinafter also referred to as a “Group III-nitride semiconductor” or “III-nitrides”) have been gaining attention as a material for semiconductor devices that emit green, blue or ultraviolet light. A light-emitting diode or a laser diode that emits blue light may be used for displays, for lighting and for high-density optical disk devices. A light-emitting device (which together with the acronym LED, when used herein, will for convenience also refer to both a light-emitting diode and laser diode unless otherwise specified) that emits ultraviolet radiation is expected to find applications in the field of ultraviolet curing, phototherapy, water and air purification, bio-detection, and germicidal treatment. The ultraviolet portion of the electromagnetic spectrum is often subdivided by wavelength into UVA (315-380 nm), UVB (280-315 nm) and UVC (<280 nm).
Due to the expected wide spread use of ultraviolet LED's there has been an ongoing desire to increase the light intensity and increase the available size of the LED. In many applications, such as water, air and food purification, there is an ongoing desire to provide an LED array with many high intensity LED emitters thereon. One method for achieving such an array is to combine individual distinct LED's into an array. This method is highly undesirable due to the large number of connections and the cost associated with assembly.
A preferred approach is to provide a substrate with a large number of LED's integral thereto. The present invention is directed to providing such a device, which was previously unavailable, and a method for manufacturing the device.
U.S. Pat. No. 6,410,940 to Jiang et al. describes the formation of an LED array wherein LED's are formed on a substrate. As illustrated in FIG. 1A therein an n-contact is in electrical contact with an n-type layer and a p-contact is in electrical contact with a p-type layer. As well understood in the art the n-contact and p-contact form the primary electrical contacts for electrical activation of the LED.
The configuration described in U.S. Pat. No. 6,410,940 is suitable for an LED comprising a highly conductive n-type layer. As would be readily realized from the description therein, the current must flow from the n-contact through the n-type layer and to the pillar comprising the quantum well layers and p-type layer. This application is suitable for GaN based LED's wherein the resistance of the layer is about 20Ω/□. As the aluminum content increases, such as in AlxGa1-xN, the resistance in the layer increases thereby rendering the structure insufficient. With low levels of Al, such as about 5-10 mole % aluminum, the resistance is about 60Ω/□. As the aluminum percentage increases, such as to about 55-60 mole fraction aluminum, the resistance increases to above about 250Ω/□. Even without aluminum the size of the array is limited since resistance in the n-type layer prohibits an extensive path length for current flow.
Group III nitride LEDs are difficult to manufacture for a number of reasons. For example, defects arise from lattice and thermal mismatch between the group III-Nitride based semiconductor layers and a substrate such as sapphire, silicon carbide, or silicon on which they are constructed. In addition, impurities and tilt boundaries result in the formation of crystalline defects. These defects have been shown to reduce the efficiency and lifetime of LEDs and LDs fabricated from these materials. These defects have been observed for III-Nitride films grown hetero-epitaxially on the above mentioned substrates with typical dislocation densities ranging from 108 cm−2 to 1010 cm−2 for films grown via metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE) and several other less common growth techniques. Reducing the dislocation density has accordingly become an important goal.
One way to reduce the dislocation density is based on the use of epitaxial lateral overgrowth (ELOG), which is a well-known technique in the prior art. With this method, the dislocation density can be reduced to 105 cm−2 to 106 cm−2. This method, however, has been shown to be ineffective for the growth of aluminum-containing III-Nitride based semiconductors because of the tendency for the aluminum to stick to the masked material and disrupt the lateral overgrowth. Several variations of this approach have also been demonstrated including PENDEO epitaxy, and FACELO growth. All of these techniques suffer from the same limitation as the ELOG approach for aluminum containing III-Nitride materials.
Additionally, a technique called cantilever epitaxy involves growth from pillars that are defined through etching as opposed to, for example, masking.
Several other approaches to dislocation reduction have been reported that do not involve selective area growth including inserting an interlayer between the substrate and the semiconductor layer to relieve strain, filtering dislocations by bending them into each other by controlling surface facet formation or by inserting a Group III-Nitride super-lattice layer as described in Applied Physics Letters, Jul. 22, 2002; Volume 81, Issue 4, pp. 604-606, between the buffer layer and the active layer.
Milli-watt power DUV LEDs on sapphire substrates with AlGaN multiple quantum well (MQW) active regions have been previously reported for the UVA, UVB and the UVC regions. The LED design used in the prior art benefited from several key innovations, namely: (1) the use of pulsed atomic layer epitaxy (PALE) to improve the quality of the buffer AlN layer; (2) the use of a PALE deposited AlN/AlxGa1-xN, short-period super-lattice layer insertion between the buffer AlN and the n-contact AlGaN layer for controlling the thin-film stress and mitigating epilayer cracking; and (3) a p-GaN/p-AlGaN hetero-junction contact layer for improved hole injection.
To date, under a cw-pump current of 20 mA, the average output powers for state-of-the-art UVC LEDs are about 1 mW. These LEDs typically have effective areas ranging from approximately 200 μm×200 μm to 300 μm×300 μm with various geometrical shapes demonstrated. Due to the poor thermal conductivity of the sapphire substrates, the output power quickly saturates at pump currents around 40-50 mA. At 20 mA pump current, the device lifetimes (50% power reduction) are approximately 1000 h for packaged devices that are flip-chipped to a heat sink. Without being constrained by theory, the key reasons for this power/lifetime limitation are the dislocations in the active region and the excessive heating due to the high device series and poor thermal conductivity of sapphire. Unfortunately, many commercial applications require the output powers and lifetimes to be significantly better than the best values reported to date.
Currently, several research groups are actively developing low-defect density AlN substrates to improve the power-lifetime performance of the deep UV LEDs. There are reports on a new air-bridge-assisted, high-temperature (1500° C.) lateral epitaxy approach to deposit 12-μm thick, high-quality AlN layers over SiC substrates as templates for the DUV LEDs. Pulsed lateral overgrowth (PLOG) of AlxGa1-xN has previously been demonstrated as an approach for depositing 15-20 μm thick AlxGa1-xN over basal plane sapphire substrates. Instead of the high temperature approach, a pulsed growth mode at 1150° C. was used to enhance Al-precursor mobilities over the growth surface. These pulsed, laterally overgrown (PLOG), AlxGa1-xN layers show a significantly reduced number of threading dislocations (˜107 cm−2) in the lateral-overgrowth regions, which enabled demonstration of optically-pumped lasing at 214 nm. In previous reports, the PLOG AlxGa1-xN was grown either from shallow (˜0.3 μm) trenched sapphire or from thin AlN etched templates (˜0.3 μm).
There are a number of reports of deep ultraviolet and visible light emitting diodes on sapphire, SiC or bulk GaN substrates using group III nitrides quantum wells in the active region. Conducting SiC and HVPE GaN substrates have the advantage of good thermal conductivity and allow for vertical conduction geometry. Unfortunately, these substrates are highly absorbing in the deep UV. Sapphire is a preferred substrate for UV LED's due to the improved light extraction efficiency yet under DC operation sapphire suffers from excessive self-heating due to the relatively high operating voltage and poor thermal conductivity of the substrate. There is a strong desire in the art to overcome the deficiencies, particularly, with regards to heat management.
However, there remains a need for a higher quality, more reliable, more robust, deep UV light-emitting diodes and laser diode arrays.